Proposals have been made in the past for various braking concepts such as anti-lock control, traction control, vehicle-body stability maintaining control, inter-vehicle distance maintaining control, and urgent brake assist control. The anti-lock control is used to prevent a locked condition of a wheel through automatic control of a brake fluid pressure supplied to the wheel during depression of the brake pedal which is established for braking or decelerating the wheel. The traction control is designed to prevent idle rotation of a driving wheel by automatic control of brake fluid supplied to the wheel during depression of the acceleration pedal for accelerating a vehicle. The vehicle-body stability maintaining control prevents extreme over-steering operation or under-steering operation during a steering operation through automatic control of brake fluid supplied to the wheels. The inter-vehicle distance maintaining control is designed to maintain a fixed distance between a proceeding vehicle and a subsequent vehicle during a constant speed travel of the latter vehicle through automatic control of hydraulic brake pressure applied to the wheel. The urgent brake assist control automatically generates a larger braking force than a usual braking force corresponding to depression of the brake pedal when the brake pedal is depressed rapidly. Some of these braking concepts have been put into practical use.
To establish the foregoing brake concepts, a conventional hydraulic brake control system has been designed to include a wheel brake for applying a hydraulic braking force to a wheel in response to hydraulic pressure supplied to the wheel brake, a reservoir in which is stored an amount of brake fluid under atmospheric pressure, a main hydraulic pressure source fluidly connected to the reservoir for generating a high hydraulic pressure in response to a force applied to an operating member of the vehicle, an auxiliary hydraulic pressure source fluidly connected to the reservoir for generating another high hydraulic pressure, and a control valve fluidly connected to the main hydraulic pressure source, the auxiliary hydraulic pressure source, the wheel brake, and the reservoir. The control valve serves to establish a supply and drain of the brake fluid under pressure to the wheel brake from either the main hydraulic pressure source and the auxiliary hydraulic pressure source. Such a conventional hydraulic brake control system is disclosed in, for example, U.S. Pat. Nos. 4,869,560, 4,310,201, and 4,565,411.
In U.S. Pat. No. 4,869,560 and U.S. Pat. No. 4,565,411, the auxiliary hydraulic pressure source is in the form of an accumulator and a pump which supplies brake fluid under pressure to the accumulator from a reservoir. In U.S. Pat. No. 4,310,201, the auxiliary hydraulic pressure source is in the form of an accumulator, a pump supplying brake fluid under pressure to the accumulator from a reservoir, and a regulating valve reducing the high pressure from the accumulator to a set value.
In the conventional hydraulic brake control system, in situations where the brake fluid is applied to the wheel brake when the driver does not depress the brake pedal upon establishment of traction control, vehicle-body stability maintaining control, inter-vehicle distance maintaining control at a constant speed travel of the vehicle and so on, the hydraulic pressure is set to be applied from the auxiliary pressure source to the wheel brake. The traction control or the vehicle-body stability maintaining control requires a rapid increase or a dynamic rise of the hydraulic pressure in the wheel brake, whereas the inter-vehicle distance maintaining control requires a gradual increase or gentle rise of the hydraulic pressure in the wheel brake. Upon introduction or supply of the brake fluid to the wheel brake from the auxiliary hydraulic pressure source, the supply of hydraulic pressure to the wheel brake is established at a high speed. Thus, to establish the gradual increase or gentle rise of the hydraulic pressure in the wheel brake, the passage between the auxiliary hydraulic pressure source and the wheel brake has to be opened and closed in an intermittent manner by switching the control valve at high cycles.
However, a large hydraulic impulse is generated whenever the passage through which the hydraulic pressure of high magnitude is intermittently opened and closed by the control valve. The resulting vibration can cause undesirable noise generation.
A need exists for a hydraulic brake control system for use in a vehicle which is free from the foregoing disadvantage and drawback.
It would be desirable to provide a vehicle hydraulic brake control system which is capable of adjusting a flow rate of a brake fluid supplied to the auxiliary hydraulic pressure source to a wheel brake without switching a control valve at high cycles.